A method for producing a single transistor storage cell is disclosed in the pending patent application of Kurt Hoffmann, "Process for the Production of a Single Transistor Memory Cell", Ser. No. 934,263 filed Aug. 16, 1978, and incorporated herein by reference. In a semiconductor crystal of first conductivity type, two zones of the opposite conductivity type and separated by a strip or portion of first conductivity type are produced as well as a depression limited in depth by converging plane surfaces situated in the region of these two zones. The two zones within the depression are produced such that they reach the semiconductor surface. Also, the pn junctions of the two zones are designed differently in relation to the portion of the semiconductor crystal exhibiting the original conductivity type such that the capacitance of the storage cell to be produced is at least predominantly provided by only one of the two pn junctions. Finally, within the depression or recess, an insulating layer is provided whereon a gate electrode, capacitively controlling the two pn junctions is provided. On a plane portion of the surface of the semiconductor crystal, first two continuous zones with different depths and of conductivity type opposite to that of the semiconductor crystal are produced and then the depression is produced at the same portion of the semiconductor surface such that the two zones are separated from one another by the depression.
Such a method is used for producing a V-MOS single transistor storage cell, known per se. Such a storage cell merely consists of a MOS-field effect transistor produced in a depression or recess of the semiconductor surface which exhibits a V-shaped profile. Due to this technique, the store cell requires little space and therefore permits the production of storage matrices with a high bit-density. The method described in the Hoffmann application now has the objective to simplify the method known for the production of V-MOS transistors operating with an epitaxial deposition of semiconductor material such that the epitaxy can be spared.
A preferred embodiment of the method mentioned proceeds such that an area A is brought into contact with a doping material which produces the opposite conductivity type in relation to that of the semiconductor crystal at a plane surface portion of the semiconductor crystal such that the doping material there effects a re-doping of the semiconductor crystal to a depth T while forming a re-doped area U.sub.1 corresponding with the shape and magnitude of area A. Moreover, a second area B adjoining the area A from the exterior is brought into contact with a doping material producing the opposite conductivity type in relation to that of the semiconductor crystal such that the doping material there effects a re-doping of the semiconductor crystal to a depth of t&lt;T while forming re-doped area U.sub.2 determined by the shape and size of area B. Finally, the depression V is produced in the areas A and B of the semiconductor surface such that at least two separate zones Z.sub.1 and Z.sub.2 are formed from the re-doped areas U.sub.1 and U.sub.2. One of the zones reaches the semiconductor surface in the depression V alongside of its edge and the other zone reaches the depression approximately at its deepest point.
The following variations of this method are additionally important for the following illustrations:
1. The area A has the shape of a square or retangle, and the area B has the shape of a strip adjoining the area A all around and having the same width, and the depression V has the shape of a reversed square pyramid whose edge surrounds the area A in parallel to its borders within the area B.
2. The area A has the shape of a rectangular strip. Additionally, pairs of areas B are provided such that the areas B also of a square or rectangular design can be represented on one another by mirroring with respect to the longitudinal symmetrical axis of area A, so that one edge of each of the areas B coincides with one portion of the longitudinal edge of area A. Two possibilities concerning the production of the depression V exist here:
(a) A depression V, having the shape of a reversed pyramid, is produced per area pair B such that the depression V orthogonally intersects the area A with two edges.
(b) A depression V, exhibiting the shape of a symmetrical reversed roof and having a length which is at least equal the length of area A, is produced and oriented in relation to the areas A and B such that the two edges of the depression V are in the areas B in parallel to the symmetrical axis of area A.
Details regarding the production of the depressions having the V-shaped converging limiting walls are contained in the Hoffmann application. At this point it is noted that the etching velocity is different in accordance with the various directions in the semiconductor crystal. This etching velocity thus behaves like a tensor, and that with a suitable orientation of the crystal surface in relation to the (111)-planes of the semiconductor grid and with corresponding orientation of the windows of the etching masks and also with the utilization of a not too rapidly working etching means, such depressions V are spontaneously formed. A suitably oriented crystal surface is identical with a (100)-plane in semiconductor materials crystallizing in accordance with the diamond grid or grids related thereto, whereas the etching masks to be utilized for the production of the depressions are designed such that the edges of their windows run in parallel to the (111)-planes. A suitable etching compound for the production of the depression is, for example, thinned KOH. A correspondingly perforated SiO.sub.2 layer can be used as the etching mask, for example, and which is applied on the surface of the semiconductor crystal, for example, by means of sputtering-on. The surface of the crystal is provided with the corresponding windows with the aid of a known photolacquer etching technique.